Allow loaders to register partial state (meta task)

Loaders performing long-running tasks should be able to record a partial state when they encounter a transient failure (for instance, for SVN loaders, a connection reset or the temporary unavailability of the target server).

This tasks records the refactorings needed to allow this.

Related swh-devel conversation : https://sympa.inria.fr/sympa/arc/swh-devel/2016-06/msg00010.html

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Migrated from T533 (view on Phabricator)